You have to be subtle when letting people know about the zombie-infested valley. You don't want mass panic, armies, chaos. Spread the softest of whispers in closed places it won't escape — virality is your enemy in more than one way — and take in the volunteers. There will always be volunteers.

Equipment is the keystone of effectiveness. Your hunters will expect weapons and supplies. Give them weapons and supplies and implanted medical sensors and position trackers and everything else they won't ask for but will be just as critical. You don't want to overburden them with supplies. You will have nearby depots, supply drones, and small helicopters for quick evacs. Offer training to those who want it.

Drop them in the valley. Ideally not in the middle of a zombie group.

Fly away.

Wait.

They will kill zombies: that's the easy part. Most volunteers won't have asked for training and wouldn't have needed it anyway. A well-armed group of humans will have a significant advantage over a much larger number of zombies for as long as their ammunition lasts. But initial morale is important, so you will want to wait a couple of days (this will usually mean a dozen zombie deaths or so for each hunter) before telling them that zombies retain self-awareness but not choice; until you tell them this it's very unlikely any of your hunters will have realized that the zombies are growling in horror and despair.

Wait.

Some of your hunters will be unable to continue. Some will force themselves or try. You will want to keep air transport near and close real-time monitoring of their medical implants. As soon as a hunter's readings indicate approaching psychological trauma or, despite your watchfulness, trauma already searing body and brain evacuate them at once to your medical facilities. Identify the group that struggles and do not resupply their ammunition.

Those will be the minority in any case. All of your hunters being volunteers, most of them will thrive under the circumstances. Your medical sensors will be easily able to identify in their neurochemistry the markers of increasing joy. Do not evacuate them from the valley, ever. Do not resupply their ammunition. Ever.

Remember to reinfest the valley as needed, and don't beat yourself up about the necessary lies. Every trap involves deceptions, sometimes layers of it, and ours is at least for a better cause than most. But never shut down your own implants. And if somebody comes to take you to the valley lie to yourself and believe the lie.